Unit 07: Design Patterns and Frameworks (1/3)


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Unit 07: Design Patterns and Frameworks (1/3)

  1. 1. Unit 7: Design Patterns and Frameworks Web presentation layer patterns  The Model-View-Controller (MVC) architectural pattern  Catalog of MVC-based patterns  Other patterns Web presentation-business integration patterns Business layer architectural patterns MVC Web frameworksdsbw 2011/2012 q1 1
  2. 2. The MVC Architectural Pattern Divides an interactive application into three components/levels: Model:  Contains the functional core of the application  Encapsulates the appropriate data, and exports procedures that perform application-specific processing View:  Displays information to the user  Obtains the data from the model  Different views present the model’s information in different ways Controller:  Accepts user input as events  Translates events to service requests for the model or display requests for the viewdsbw 2011/2012 q1 2
  3. 3. The “Classical” MVC Pattern Observer * update() 1 Modeldataattach( ob : Observer ) Viewdetach( ob : Observer ) Controllernotify() 1 1getData() initialize( m : Model ) initialize( m : Model, v : View )service() makeController() handleEvent() activate() update() display() update()dsbw 2011/2012 q1 3
  4. 4. Catalog of MVC-Based Web Presentation Patterns Controller Patterns:  Page Controller  Front Controller  Application Controller  Intercepting Filter View Patterns  View Helper  Composite View  Transform View Composite Patterns:  Dispatcher View  Service to Workerdsbw 2011/2012 q1 4
  5. 5. Page Controller Each Page Controller component act as the controller for a dynamic page on the Web site. The basic responsibilities of a Page Controller are:  Decode the URL and extract any form data  Invoke model components to process the request data  Determine which view should display the result page and forward the model information to itdsbw 2011/2012 q1 5
  6. 6. Page Controller: How It Worksdsbw 2011/2012 q1 6
  7. 7. Page Controller: When to Use It Page Controller works particularly well in a site where most of the controller logic is pretty simple: : User : System service(parameters) result Variant: Page Controller and View implemented by the same Server Page: (Model 1)dsbw 2011/2012 q1 7
  8. 8. Front Controller (Model 2) Problem: The system requires a centralized access point for request handling. Without a central access point, control code that is common across multiple requests is duplicated in numerous places. When control code is intermingled with view-creation code, the application is less modular and cohesive. Forces:  You want to avoid duplicate control logic.  You want to apply common logic to multiple requests.  You want to separate system processing logic from the view.  You want to centralize controlled access points into your system Solution: Use a Front Controller as the initial point of contact for handling all related requests. The Front Controller centralizes control logic that might otherwise be duplicated, and manages the key request handling activities .dsbw 2011/2012 q1 8
  9. 9. Front Controller: Class Diagram 1dsbw 2011/2012 q1 9
  10. 10. Front Controller: Sequence Diagramdsbw 2011/2012 q1 10
  11. 11. Front Controller + Application Controller An Application Controller has two main responsibilities: deciding which domain logic to run and deciding the view with which display the response. It is useful for designing complex use cases with definite rules about the order in which pages should be visited and different views depending on the state of objects. Separating the Application Controller from the Front Controller allows improving the modularity of the system, while enhancing its reusability. The Application Controller component is not a Server Pagedsbw 2011/2012 q1 11
  12. 12. Front Controller + Application Controller (cont.)dsbw 2011/2012 q1 12
  13. 13. Application Controller with Mapper Application Controller: Uses Mapper to resolve an incoming request to the appropriate action and view, to which it delegates or dispatches. Mapper: Uses a Map to translate an incoming request into the appropriate action and view. A Mapper acts as a factory Map: Acts as a dictionary or registry of references to target resources. Target: A resource that helps fulfill a particular request (view, command)dsbw 2011/2012 q1 13
  14. 14. Application Controller with Mapper (cont.)dsbw 2011/2012 q1 14
  15. 15. Application Controller with Mapper (cont.)dsbw 2011/2012 q1 15
  16. 16. Intercepting Filter Problem: You want to intercept and manipulate a request and a response before and after the request is processed in order to determine if:  The client has a valid session  The request path violates any constraint  You support the browser type of the client  The client has used a special encoding to send the data  The request stream is encrypted or compressed Forces  You want centralized, common processing across requests  You want pre and postprocessing components loosely coupled with core request-handling services.  You want pre and postprocessing components independent of each other and self contained to facilitate reuse. Solution: Use an Intercepting Filter as a pluggable filter to pre and postprocess requests and responses. A Filter Manager combines loosely coupled filters in a chain, delegating control to the appropriate filter.dsbw 2011/2012 q1 16
  17. 17. Intercepting Filter: Structure, Participants and Responsibilities FilterManager: Manages filter processing. It creates the FilterChain with the appropriate filters, in the correct order, and initiates processing. FilterChain: Ordered collection of independent filters Filter: Component that performs a specific pre and/or postprocessing task. Target: The resource requested by the client.dsbw 2011/2012 q1 17
  18. 18. Intercepting Filter: Sequence Diagramdsbw 2011/2012 q1 18
  19. 19. Java Filters: Example Motivation: HTML forms that include a file upload use a different encoding type than that of basic forms. As a result, form data that accompanies the upload is not available via simple getParameter() invocations. The following filter preprocesses requests to translate the encoding type into a single consistent format that makes all form data available as request attributes: public class MultipartEncodeFilter extends javax.servlet.Filter { public void doFilter(javax.servlet.ServletRequest request, javax.servlet.ServletResponse response, javax.servlet.FilterChain filterChain) throws java.io.IOException, javax.servlet.ServletException { String contentType = request.getContentType(); if (contentType.startsWith("multipart/form-data") ) { For each pair (attributeName, value) obtained from the request do request.setAttribute(attributeName, value) } filterChain.doFilter(request, response); } }dsbw 2011/2012 q1 19
  20. 20. View Helper Problem: You want to separate a view from its processing logic.  JSP and other template-based views allow embedding scriptlet code  Embedded scriptlet code cannot be reused easily  Embedded scriptlet code often acts as control code or performs view preparation activities, such as content retrieval.  Mingling control logic, data access logic and formatting logic leads to problems with modularity, reuse, maintenance, and role separation. Forces:  You want to use template-based views, such as JSP.  You want to avoid embedding program logic in the view.  You want to separate programming logic from the view to facilitate division of labor between software developers and web page designers Solution: Use Views to encapsulate formatting code and Helpers to encapsulate view-processing logic.  A View delegates its processing responsibilities to its helper classes, implemented as POJOs, custom tags, or tag files.  Helpers serve as adapters between the view and the model, and perform processing related to formatting logic, such as generating an HTML table.dsbw 2011/2012 q1 20
  21. 21. View Helper: Structure Helper: Encapsulates processing logic for generating and formatting a View. A helper typically adapts a PresentationModel for a view or provides access to the raw data of the PresentationModel PresentationModel: Holds the data retrieved from the business service, used to generate the View. It can be either a Business Delegate or a DTOdsbw 2011/2012 q1 21
  22. 22. View Helper: Sequence Diagramdsbw 2011/2012 q1 22
  23. 23. View Helper: Example with JavaBean and JSTL<%@ taglib uri=http://java.sun.com/jsp/jstl/coreprefix="c" %><jsp:useBean id="welcomeHelper" scope="request"class="WelcomeHelper" /><HTML><BODY bgcolor="FFFFFF"><c:if test = "${welcomeHelper.nameExists == true}"><center><H3> Welcome <b><c:out value=${welcomeHelper.name}/></b><br><br></H3></center></c:if><H4><center>We are happy for your visit!</center></H4></BODY></HTML>dsbw 2011/2012 q1 23
  24. 24. Composite View Problem: You want to build a view from modular, atomic component parts that are combined to create a composite whole, while managing the content and the layout independently. Forces  You want common subviews, such as headers, footers and tables reused in multiple views, which may appear in different locations within each page layout.  You have content in subviews that frequently changes or might be subject to certain access controls, such as limiting access to users in certain roles.  You want to avoid directly embedding and duplicating subviews in multiple views which makes layout changes difficult to manage and maintain. Solution: Use Composite Views that are composed of multiple atomic subviews. Each subview of the overall template can be included dynamically in the whole, and the layout of the page can be managed independently of the content.dsbw 2011/2012 q1 24
  25. 25. Composite View: Structure Template: Represents the page layout ViewManager: Uses a Template to enforce a layout into which it places the appropriate content.  A simple ViewManager might use the standard JSP include tag (<jsp:include>) to include SimpleView segments into a Template.  A more sophisticated ViewManager might use POJOs or custom tag helpers to provide content and layout management in a more comprehensive and robust manner.dsbw 2011/2012 q1 25
  26. 26. Composite View: Sequence Diagramdsbw 2011/2012 q1 26
  27. 27. Transform View Problem: You want a view that processes domain data element by element and transforms it into HTML. Forces: The data returned by the business layer is either in XML or in something automatically transformable to it Solution: Use a Transform View that transforms directly from domain-oriented XML into (X)HTML by using XSLT (eXtensible Stylesheet LanguageTransformations). Normally XSLT does this by transforming each XML element into an (X)HTML element.dsbw 2011/2012 q1 27
  28. 28. Dispatcher View Problem: You want a view to handle a request and generate a response, with little or no business processing performed before rendering the view. Forces:  You have static views  You have views generated from an existing presentation model  You have views which are independent of any business service response  You have limited business processing Solution: Use Dispatcher View with views as the initial access point for a request. Business processing, if necessary in limited form, is managed by the views.dsbw 2011/2012 q1 28
  29. 29. Dispatcher View: Structure BusinessHelper: Helps the View initiate business processing to handle a request BusinessService: Encapsulates the business logic and business state. A remote business service is accessed via a Business Delegate PresentationModel: Holds the data retrieved from the business service (DTO).dsbw 2011/2012 q1 29
  30. 30. Dispatcher View: Sequence Diagramdsbw 2011/2012 q1 30
  31. 31. Service To Worker Problem: You want to perform core request handling and invoke business logic before control is passed to the view Forces:  You want specific business logic executed to service a request in order to retrieve content that will be used to generate a dynamic response.  You have view selections which may depend on responses from business service invocations.  You may have to use a framework or library in the application Solution: Use Service to Worker to centralize control and request handling to retrieve a presentation model before turning control over to the view. The view generates a dynamic response based on the presentation model.dsbw 2011/2012 q1 31
  32. 32. Service To Worker: Structuredsbw 2011/2012 q1 32
  33. 33. Service to Worker: Sequence Diagramdsbw 2011/2012 q1 33
  34. 34. References Books  ALUR, Deepak et al. Core J2EE Patterns: Best Practices and Design Strategies, 2on Edition, Prentice Hall PTR, 2003.  FOWLER, Martin Patterns of Enterprise Application Architecture, Addison Wesley, 2002. Webs  www.corej2eepatterns.com  java.sun.com/blueprints/corej2eepatterns/  java.sun.com/blueprints/guidelines/designing_enterprise_appli cations_2e/web-tier/web-tier5.htmldsbw 2011/2012 q1 34